1. Technical Field
The present invention relates to a driver and dimmer/switch therefore for an array of light emitting diodes (LED's).
2. Brief Description of the Prior Art
LED is abbreviation of “Light Emitting Diode”, which is a small electronic device that lights up when an electric current is passed through it. The term diode refers to a family of two-pin semiconductor devices. The current can pass through them only in one direction. The first LED's were red. They were introduced to the market decades ago. The early red LED's quickly found applications as tiny indicators on audio equipment, TV's, and even digital wrist watches. Later, LED's were used as seven-segment display modules, and the first pocket calculators used them. Years of research has introduced all sorts of colorful LED's to the market. The most common LED's are red, green, yellow, blue, and orange. The color of LED is due to the material used in the LED chip not just the color of the package. In the past several years, the LED market has seen a big jump in the brightness of the LED's, and white LED's have been introduced that produce enough light that they have been used in cars and general lighting.
The main advantages of LED's are long life span (some exceeding 100,000 hours), and high efficiency compared to small tungsten or incandescent lights. Additionally, they generate very little heat when they are operated at the rated current. They can also take a harsh environment, as there is no filament in them. The disadvantages (at least when compared to 110V tungsten light bulbs) are that they can not directly replace incandescent lamps, and, a single LED is very small and generally cannot generate enough light to light up a room. Therefore, the LED's for generating a large amount of light are typically used in clusters. Some designers used them in series strings, some use them in parallel strings, and some use them in a combination of series and parallel strings.
The LED's are normally used in constant-current circuits. The early LED's required only 10 milliamperes to operate. Many new ultra-bright white LED arrays require a current of 750 milliamperes to operate at maximum brightness.
The term “LED driver” refers to any kind of electronic circuit that produces the current and voltage necessary to turn on a specific LED or cluster of LED's. For example, some LED drivers can take as input the 12VDC from a car battery, and generate enough current to turn on a combination cluster of 20 LED's used in a tail light. Another example is an LED driver that turns on a combination of LED clusters used as the backlighting for flat panel LCD displays (the LED's have effectively replaced the fluorescent back lighting).
The LED driver for commercial and residential lighting is different because the input voltage will typically be 110 volts AC. This voltage needs to be converted to DC and also it needs to be regulated such that it does not feed more than the necessary amount of current to the LED's. If the LED's are driven by higher currents and voltage than their rated values, their life span will significantly shorten or they may even burn out quickly.
Currently, lighting fixture companies use LED drivers for fixtures such as chandeliers that are so large they can barely fit into the ceiling or fixture canopy. The drivers also do not have any onboard or external dimmer. It has been proposed to use a conventional incandescent 110 volt AC dimmer for dimming LED's. This is an awkward way of solving the problem because two units have to be installed, one in the ceiling and one in the wall outlet for the fixture. In addition, there are compatibility issues between LED drivers and incandescent dimmers.
Published US application 2004/0212321 discloses an LED driver configured to provide power from an Ac 110 volt circuit to a plurality of LED's. The driver gets its power from rectified standard AC voltage. Further, a conventional AC dimmer is used for dimming functionality.
Published US application 2006/0113975 discloses controlling output current of a DC/DC converter. While this circuit could be employed in an LED driver, it does not disclose the technology of the present invention.
U.S. Pat. No. 6,940,733 discloses a power supply using a frequency modulated pulse train for optimal power conversion. The circuitry of the present invention employs a fixed frequency.
U.S. Pat. No. 7,145,295 discloses a simple design for controlling light emitting diodes. While this design could be used for dimming LED's, it does not disclose a technology as how to power, dim, and switch LED's on/off in an offline application that could also be fit in an AC outlet for lighting applications.
Published Data Sheet HV9910 titled “Universal High Brightness LEDDriver” by Supertex, Inc, 1235 Bordeaux Drive, Sunnyvale, Calif., 94089, discloses a PWM high efficiency LED driver control IC. It allows efficient operation of High Brightness (HB) LED's from voltage sources ranging from 8VDC up to 450VDC. The HV9910 controls an external MOSFET at fixed switching frequency up to 300 kHz. The frequency can be programmed using a single resistor. The LED string is driven at constant current rather than constant voltage, thus providing constant light output and enhanced reliability. The output current can be programmed between a few milliamps and up to more than 1.0 A. The HV9910 uses a rugged high voltage junction isolated process that can withstand an input voltage surge of up to 450V. Output current to an LED string can be programmed to any value between zero and its maximum value by applying an external control voltage at the linear dimming control input of the HV9910. The HV9910 provides a low-frequency PWM dimming input that can accept an external control signal with a duty ratio of 0-100% and a frequency of up to a few kilohertz.